Benzo(a)pyrene (BP) may be metabolized to nontoxic products or to the carcinogen, BP-7,8-diol-9,10-epoxide. The direction of BP metabolism depends on the presence of specific forms of enzymes in individuals exposed to chemical carcinogens. Monoclonal antibodies, (MAbs) are specific probes for particular antigenic determinants and are useful tools for identification of particular isoenzymes. We prepared MAbs to cytochrome P-450 and epoxide hydrolase (EH), which are two key components of the system responsible for the metabolism of BP. The MAbs bind to human liver cytochrome P-450 but do not immunoprecipitate or inhibit enzymatic activity; the last observation suggests that the MAbs were raised against an antigenic determinant whose binding does not interfere with the functioning of the catalytic site. The MAbs against EH bind and precipitate only the form which was used as antigen but do not interact with other forms.